Theories and methods from the prospective memory literature were used to anticipate how individuals would maintain and retrieve intentions in a continuous monitoring dynamic display task. Participants accepted aircraft into sectors and detected aircraft conflicts during an air traffic control simulation. They were sometimes required to substitute new actions for routine actions when accepting aircraft traveling at certain speeds or altitudes, or with certain call signs. In Experiment 1, prospective memory error increased with intent to deviate from strong compared to weak routine, and this effect was larger for altitude intentions compared to speed intentions. In addition, errors increased when intentions were general compared to specific. Participants also missed more conflicts when deviating from strong compared to weak routine. In Experiment 2, errors increased for intentions nonfocal to ongoing tasks compared to focal, and this effect was larger for altitude intentions compared to call sign intentions. Participants were slower to accept aircraft when holding nonfocal compared to focal intentions, and slower to accept aircraft and detect conflicts when holding focal intentions compared to no intentions. These findings are consistent with theories that assume that individuals allocate limited-capacity attentional resources to prospective memory tasks. Increased error for altitude intentions, together with the effect of routine strength, suggest a vulnerability to error with increased strength of association between prospective memory cues and competing ongoing tasks. Ongoing tasks that focus attention on cues may sometimes impair, rather than benefit, intention retrieval. (Contains 6 figures.)